Medicaments for treating respiratory disorders are frequently administered as dry powder formulations through the mouth and nose. Dry powder inhalation devices, or inhalers, are used in the administration of these drugs, inhalation by the patient resulting in uptake of a specified dosage of medicament through the nose or mouth.
The drug may be stored as a dry powder within a reservoir in the body of the inhaler, a metering chamber being utilised to administer a specified dose of medicament. Alternatively, more sophisticated inhalation devices employ medicament carriers, such as individual capsules or blister packs/strips containing defined doses of powdered drug.
It is also known to use for such therapy medicaments which are contained in an aerosol and are administered to a patient by means of an inhalation device. The aerosol containers used in such inhalation devices are designed to deliver a predetermined dose of medicament upon each actuation and are known as Metered Dose Inhalers (MDIs); see Peter Byron, Respiratory Drug Delivery, CRC Press, Boca Raton, Fla. (1990) for a general background on this form of therapy.
Mechanical and electrical dose indicating devices, which typically count the number of doses delivered from or remaining in the medicament dispenser, are frequently used in dry powder inhalers and MDIs to enable patients to determine how much medicament is available in the dispenser for future use.
One problem associated with both mechanical and electrical dose counters is that they may, on occasions, give false readings. Thus the electronic dose counter disclosed in U.S. Pat. No. 5,020,527 employs a mechanical trigger, which may be actuated without release of the medicament, thereby giving a false reading. Other forms of triggers are also known, particularly the use of sensing means to detect actuation of the dispenser.
U.S. Pat. No. 5,544,647 discloses a range of sensors which may be used to detect and record actuation of MDIs. Typical sensors include pressure switches which are sensitive to either physical contact with a movable element of the device or to the inward breath of the patient. Other examples include sensors which are responsive to light, such as reflected or emitted light, or optical sensors which recognise reference points on the container or housing. These sensors are designed to detect movement of the aerosol container following actuation of the device by being aligned to a reference point or a light source, such as a LED. Sensors which are responsive to electromagnetic radiation, such as fluctuations in electromagnetic fields caused by movement of the container, are also disclosed within this document.
WO 95/07724 describes a dry powder inhaler with an electronic dose counter which employs a series of sensors to validate actuation of the inhaler. The inhaler uses magnetically responsive proximity reed switches to detect medicament loading within the device, and a thermistor sensor to detect temperature changes due to inhalation by the patient.
A general problem encountered with dose counters in the art, such as those described above, is that they are dependent upon sensors which do not directly detect the medicament release from the dispenser. All of the sensors used in the art are arranged to sense some feature associated with actuation of the device, such as movement of the container/housing or pressure/temperature changes due to inhalation. This indirect method of sensing medicament release can lead to false readings being registered on the dose counter, if for example the sensor is activated without release of medicament. Accidental activation or triggering of the sensor may, for example, result from partial movement of the container or from interference with light received by the sensor. Similarly, a blockage or only partial release of the drug from either an MDI or dry powder inhaler would lead to a false reading registering on the dose counter.